High-frequency differential transformer



Nov. 11, 1930. M. vos 1,781,308

HIGH FREQUENCY DIFFERENTIAL TRANSFORMER Filed May 29, 1929 M/vewfa 1L Patented Nov. 11, 1930 UNITED STATES PATENT OFFICE MAURITZ VOS, OF STOCKHOLM, SWEDEN, ASSIGNOR TO TELEFONAKTIEBOLAGET L. M. ERICSSON, OF STOCKHOLM, SWEDEN, A COMPANY OF SWEDEN HIGH-FREQUENCY DIFFERENTIAL TRANSFORMER Application filed May 29, 1929, Serial No. 367,093, and in Sweden May 30, 1928.

scribed with reference to the accompanying drawing. Figure 1 shows diagrammatical- 1y for comparison the usual construction of a differential transformer adapted for ordinary speech current-s. Figure 2 shows diagrammatically an embodiment of a high fre-- quency differential transformer according to the invention. Figure 3 shows diagrammatically a detail of the one part of the transformer according to Figure 2. Figure 4 shows a section of the winding on a larger scale, and Figure 5 is a side elevation of one of two separate coils forming together the transformer, said coil being shown in a partial wound condition.

Differential transformers for telephone purposes must be constructed to meet the following general requirements:

1. The coupling coefficient between the different windings must be approximately equal to 1.

2. The transformer must be connected to the line in such a manner that in all respects it is symmetrical in relation to earth.

3. The ohmic and iron losses must be small.

These requirements are easy to fulfill in respect of differential transformers for lower frequencies such as speech frequencies. In the design of such a differential transformer for ordinary telephone currents, as shown diagrammatically in Figure 1, the primary winding is divided into four partial windings A A A A, which, connected in series, two partial windings being in-' cluded in each of the branches of the incoming or outgoing line L, to which the differential transformer is connected by the terminals 1. Inserted between the opposite terminals 2 of the primary winding is a balance impedance N having the same impedance value as the line. The secondary winding consists of two partial windings B 13 connected in series each of which is coupled inductively to one partial winding A and A, respectively or A and A respectively in each of the line branches. As the use of an iron core in this case does not result in any substantial losses or other inconveniences, the coupling coefficient between the windings may be made sufficiently tight without difficulty. The two amplifiers of the two-wire repeater operating in opposite directions of propagation are connected up to the middle terminals 3 of the primary winding and the terminals 1 of the secondary winding respectively. The letter I designates for instance the input impedance of one of said amplifiers, while U designates the output impedance of the otheramplifier. The partial windings included in different branches of the line L, evidently, form in relation to each other a condenser interconnected between the branches and having a comparatively large capacity. In a transformer for currents of low frequency said capacity does not, however, disturb the speech transmission to any noticeable degree.

On the other hand, certain constructive difficulties arise when designing a differential transformer for high frequency currents. In this case the following additional requirements must be fulfilled by the differential transformer 4. The transformer must not introduce any capacity between the branches of the line because such a capacity would represent a short-circuit for the high frequency currents.

5. The high degree of coupling required between the windings must be brought about without the use of iron cores as otherwise the iron losses would be too great.

According to the invention the required light coupling in the ironless differential transformer and the desired absence of capacity between the branches is brought about by composing the differential transformer of two separate coils adapted to be connected to each one of the branches of a line circuit, each of said coils consisting of two series connected primary partial windings and two sec- Ondary partial windings which four partial windings are formed b parallel conductors of a twisted quad of a four core cable, means protecting each of the coils against capacitive influence of the other coil being provided.

Figure 2 shows diagranimatically a differential transformer for high frequency designed according to the invention. The one of the two coils of the transformer comprises the two primary partial windings A A connected in series in the one line branch and the two secondary partial windings B B The second coil of the transformer similarly comp rises two primary partial windings A A connected in series in the other line branch and two secondary partial windings B 13.. Each of said transformer coils is enclosed in acover 5 and 5 respectively. The differential transformer is thus sub-divided into two parts capacitively separated from each other and included in different line branches. Each coil is mounted on a bobbin 6, Figure 5, provided with two annular coil spaces 89 separated by a partition 7. The

four partial windings of each coil are formed by four parallel stranded conductors 10 of afour-core cable or a quad wire bundle 11 the one half of which is wound in the coil space 8 and the other half in the coil space 9 on the bobbin 6. The transformer as a whole will, consequently, comprise two bobbins 6 each provided with windings as described. The

winding on of the cable 11 is performed, as

shown in Figure 5, in such a manner that the cable at its middle point 12 is pressed into a radial slot 13 in the partition 7 down to the bottom of the coil spaces. Thereupon first the one half of the cable is wound on in the appertaining coil space, while the other half in its entirety is allowed to participate in the rotation of the bobbin. After the first mentioned coil space, by way of example 8, has been thus filled by one half of the cable, the second half of the cable is wound on in the opposite direction in the other coil space 9. During the winding on of the cable the latter is twisted uniformly so that the four wires are-twisted mutually with a constant pitch. By this method all the winding parts will be in-a quite equivalent relation to each other.

By the described disposition of the coils a complete symmetry in each coil is obtained. This can be seen most clearly in Figure 3 which illustrates diagrammatically the winding arrangement for the upper coil in Figure 2. In Figure 3 two groups of four windings are shown each group representing one half of the cable 11 and each being provided in a coil space 8 or 9 respectively. The junctions 12 between the two groups correspond to the transversal part 12 of the cable squeezed into the slot 13 of the partition. Each of the partial windings A A B 2 are illustrated in Figure 3 as comprising two windings. elements connected in series said winding elements being disposed each in one of the winding spaces 8, 9 of the bobbin. By disposing the middle point of the winding in nermost in the coil all the tappings can be arranged on the outside of the coil, as shown in Figure 3. The tappings a, 0 and d, e respectively mutually corresponding to each other will then be disposed symmetrically on opposite sides of the partition 7 whereas the middle point tapping b of the primary winding half will be disposed in the middle on the outside of the coil.

By the described disposition of the four partial windings in each coil 2. coupling coeflicient between the different part windings is obtained which partically equals 1.

The above described arrangement in which the differential transformer is composed of two separate coils and made up of a twisted quad cable may, of course, also be applied in those cases when iron may be allowed to be present in the circuit. This arrangement has the advantage among other that the capacity between the line branches is reduced.

I claim:

1. A transformer for high frequency 0scillations comprising in combination two separate coils adapted to be connected to each one of the branches of a line circuit, each of said coils consisting of two series connected primary partial windings and two secondary partial windings which four partial windings are formed by parallel conductors of a twisted quad of a four-core cable, and means protecting each of the coils against capacitiveinfluence of the other coil.

2. A transformer as claimed in claim 1, characterized by the four conductors being twisted together with a uniform pitch.

3. A transformer as claimed in claim 1, characterized by the middle point of the twisted cable of each coil being disposed innermost in the coil from which point the two halves of the twisted cable are wound in opposite directions.

4. A transformer as claimed in claim 1, characterized by the middle point of the twisted cable on each coil being squeezed into a radial slot in a partition dividing the coil into two e ual parts, the two halves of the twisted cab e being wound on opposite sides of and symmetrically in relation to said partition.

In testimony whereof I aflix my signature.

MAURITZ' VOS. 

